The present invention relates to a transverse charge transfer filter.
The expression elementary transverse charge transfer filter means a system of delay stages to whose input is applied the signal to be filtered, the filtered signal being obtained by forming the sum of all the output signals of the different delay stages, multiplied by the weight factors.
A complex transverse charge transfer filter is understood to mean a group of elementary transverse charge transfer filters to whose input are applied different signals to be filtered, the filtered signal being obtained by forming the sum of the output signals of all the delay stages of the different elementary filters.
Hereinafter, the term filter will apply either to an elementary filter or to a complex filter.
Each delay stage is obtained by means of a group of MOS (Meta Oxide Semiconductor) capacitors arranged in an adjacent manner on the same semiconductor substrate, one of the capacitors being provided with an electrode responsible for reading the the charge, whilst the other capacitors are provided with electrodes responsible for controlling the charge transfer. The number of MOS capacitors used for controlling the charge transfer varies essentially with the technology of the filter and with the number of control phases used.
To avoid any break in the charge transfer between the different stages and, within each stage, between the different MOS capacitors thereof, any gaps in the arrangement of successive electrodes must be avoided. For this purpose, it is known to overlap the ends of adjacent electrodes, an insulator then being provided to prevent any contact between the electrodes in the overlap area. The overlapping of a reading electrode by two adjacent control electrodes causes a variation in the reading of the charge quantity located beneath said reading electrode.
Thus, no matter what the reading method used (reading in current with fixed voltage reading electrode or reading in voltage with floating reading electrode) and no matter what the number of control electrodes per delay stage, a variation in potential must be produced during the reading operation, on at least one of the two control electrodes surrounding the reading electrode in question, so as to ensure the charge transfer beneath said reading electrode. This variation in potential is transmitted to the reading electrode via the capacitor formed by the end of the reading electrode and the control electrode which overlap and by the insulating layer inserted between these ends.
This variation in to the reading leads to the addition of a d.c. component bias to the a.c. component resulting from the actual reading of the charge located beneath the reading electrode in question. The value of this d.c. component is dependent on the widths of the overlaps (considered in the longitudinal direction of the filter) on the reading electrode in question by adjacent control electrodes, on which control electrodes the variations in potential occur during reading. The different reading electrodes of the filter are interconnected, so that the variations corresponding to these different reading electrodes are summed.
During the manufacture of a filter, the overlap widths between electrodes are fixed for all the electrodes of the filter by the positioning, with respect to the electrodes whose ends are to be overlapped, of a mask, whose non-recessed portions serve to preserve the parts which are not to be overlapped.
A problem then occurs as a result of the fact that the positioning of the mask can vary from one filter to the next, which filters otherwise have identical constructional characteristics. Thus, if the positioning of the mask varies, the d.c. reading component also varies. However, for facilitating the acquisition of the reading signal, it is advantageous to have the same d.c. reading component among the individual filters. This applies more particularly as a result of the fact that a small variation in the positioning of the mask leads to a relatively large variation of the d.c. component.